Fix various issues and add improved test for dependency resolution

lib/laradi.rb

@@ -8,9 +8,16 @@ module Laradi
 
   module Mixin
     def dependencies(*deps)
+      result = {}
       deps.each { |dep|
-        include $laradi_injector[dep]
+        instance = $laradi_injector[dep]
+        if instance.nil?
+          raise NoDependencyError, "No dependency found for #{dep}"
+        end
+        result[dep] = instance
       }
+
+      result
     end
   end